1. Field of the Invention
The field of the invention generally relates to thermoplastic co-polyesters having improved properties mechanical and thermal properties. The field of the invention also includes methods of making the same and improved products, in particular, improved railroad products, constructed from the improved thermoplastic co-polyesters.
2. Description of the Related Art
There currently are a number of commercialized products made from co-polyesters. Co-polyester is the term applied to those polyesters whose synthesis uses more than one glycol and/or more than one dibasic acid. Thermoplastic co-polyesters generally have linear molecular structures and are able to flow freely at elevated temperatures. For this reason, thermoplastic co-polyesters are preferred for products which are produced by injection molding or other extrusion techniques, where flowability of the reactants is of paramount importance. Unfortunately, thermoplastic co-polyesters typically exhibit poor performance characteristics with respect to abrasion, tensile strength, rebound, compression set and thermal properties.
In contrast to current thermoplastic co-polyesters, thermoset urethanes have particularly good characteristics with respect to abrasion, tensile strength, rebound, compression set and thermal properties. Thermoset urethanes generally have a network structure that incorporates irreversible chemical cross-linking. The downside of thermoset urethane is that the irreversible chemical cross-linking reaction makes it unsuitable for use in injection molding and extrusion applications. Typically, thermoset urethanes are formed using a casting process. Unfortunately, casting processes require costly equipment, usually involve a large number of processing steps and are difficult to use. Casting is thus a less efficient and more expensive method of producing products as compared to injection molding and extrusion systems.
With respect to thermoplastic co-polyesters, a dial or polyol is reacted with a dimethyl terephthalate. This reaction typically takes place in large commercial reactors. As stated above, thermoplastic co-polyester, while not cross-linked, is usable in injection molding and other extrusion methods. Because of the lack of cross-linking, these materials have abrasion, tensile, and compression set properties that are not as good as thermoset polyurethane. One solution has been to use a costly thermoset urethane, which for larger products, adds to their production cost, while another has been just to accept the lesser properties that are provided by the use of a thermoset co-polyester. There also has been proposed the use of a cross-linked thermoplastic polyurethane and polyurea in U.S. Pat. No. 8,003,747.
In railroad applications thermoplastic materials may be used to construct railroad components, such as, for example, springs for draft gears, constant contact side springs, primary suspension components, bumpers, track tie pads and steering pads for bearing adapters. Injection molding or extrusion of products involves the utilization of mold components, and involves a residence time that the molding material remains in the mold or mold components. A need exists for a material that may be utilized in extrusion or injection molding processes to form articles which also reduce the amount of residence time required, making the mold components available for further use, at a sooner time interval. A need also exists for railroad components that may be produced having improved heat resistance, increased compression set or strength, improved resistance to creep, and improved rebound when used for spring, steering pad or track pad components.
There thus is a need for a thermoplastic co-polyester material which exhibits good abrasion, rebound, thermal properties and compression set characteristics which are similar to those found in thermoset urethanes. There is a need for a material that enables the articles to be produced using conventional injection molding and/or extrusion techniques, where the articles produced also are durable and lasting when in use.